Antagonism of the positive dromotropic effect of isoproterenol by adenosine: Role of nitric oxide, cGMP-dependent cAMP-phosphodiesterase and protein kinase G

We hypothesized that nitric oxide (NO) plays an important role in mediating the anti-adrenergic effect of adenosine on atrioventricular (AV nodal cond uction, In guinea-pig hearts instrumented for measurement of AV nodal condu ction time (atrium-to-His bundle, A-II, interval), the NO synthase (NOS) in hibitor, L-NMMA (100 mu M), reversibly inhibited 80% (P= 0.009, n = 6) of a denosine's anti-adrenergic action on the positive dromotropic effect of iso proterenol (0.01 mu M), In parallel studies carried out in rabbit AV nodal myocytes, intracellular mechanisms whereby NO mediates the inhibitory effect of adenosine on isopro terenol-induced A-H interval shortening were studied, Adenosine (3 mu M) in hibited isoproterenol-stimulated (0.1 mu M) I-Ca,I-L (beta-I-Ca,I-L) by 46 +/- 6% (P<0.001, n=17). Consistent with isolated heart data, the NOS inhibi tors, L-NMMA (100 mu M) and L-NNA (500 mu M) attenuated the effect of adeno sine on beta-I-Ca,I-L by 69 +/- 8% (P<0.001, n=16) and 69 +/- 7% (P<0.001, n=10), respectively. An inhibitor of NO-stimulated guanylyl cyclase LY83538 (40 mu M) reduced the inhibitory effect of adenosine on beta-I-Ca,I-L by 9 7+/-6% (P=0.004. n = 15), Similarly, the non-specific inhibitor of cAMP-pho sphodiesterases IBMX (50 mu M) decreased the anti-adrenergic effect of aden osine by 60% (P = 0.02, n = 6), whereas the extracellular application of th e nonhydrolyzeable cAMP analog 8-Br-cAMP (500 mu M) prevented this action o f adenosine, Activation of cGMP-dependent protein kinase (PKG) by CPT-cGMP (300 mu M) diminished beta-I-Ca,I-L but to a significantly smaller degree ( 16+/-4%, P=0.025, n=12) than that caused by adenosine, NO mediates the anti-adrenergic effect of adenosine on AV nodal conduction by a mechanism predominately involving activation of cGMP-dependent cAMP-ph osphodiesterase and to a lesser extent activation of PKG. (C) 2000 Academic Press.